It sounds like you've experienced relationships where partners have overridden your boundaries or pressurised you into sex (both of those things are sexual abuse, BTW) and you're having a hard time relaxing and trusting that your current partner isn't going to be the same way, even though he's never done either.
I get that and I understand the fear about being honest. When you've been abused in the past it's difficult to just throw off those guards you've built up to protect you. Sometimes it can even lead to a fear/revulsion/anxiety around sex in general which isn't a nice place to be. It's very complicated I find with sex because often the guarded reaction is to go along with something while pretending it's all fine to avoid aggression or disapproval, and that can sort of get in the way of the actual enjoyment of sex.
How long have you been with your current partner? Would it be helpful to see it as a powerful thing - that you'll tell him, and his reaction will then be yours to decide how to react to? So for example if he is horrified and sympathetic and promises to check you're OK, then that's great - a green light. But if he is dismissive or laughs about it or uses it to get what he wants, that's a red light and you have the absolute right and the power to end the relationship?
I know when I first got together with my now DH I had decided that I never wanted to be in a relationship with somebody who didn't respect my boundaries, and that being single would be preferable, so I told him about feeling like this, about not feeling I "should" say no even though he had never expressed any desire or intent to coerce or force me, and we talked about it - I can't remember how I brought it up now but it was difficult - and he absolutely categorically said 100% to just stop any time, and it didn't matter if it was in the middle of things, and that he would have no reason to be angry about it ever. I then spent a few weeks doing just that sort of to "test" it out and see how he would react 
- he never once lost patience or seemed annoyed. It was like he totally 100% believed that my body was my own and he didn't have the right to use it unless I actively and enthusiastically offered 
- I know in theory, that's a totally normal and healthy expectation, but at the time I wondered what is the catch and when is it going to run out because I don't quite trust this. We have been together 8 years. I do now feel 100% confident in just saying "Actually this isn't doing anything for me right now" (and then changing or stopping). We have found a balance too. However, I still don't really know how to explain to him that while he hears/thinks "sex" and his brain goes "Ooh a lovely intimate cuddle with orgasm" my brain thinks/hears "sex" and goes "danger" (Of pain, discomfort, of being pushed past my boundaries, of feeling forced to do something I find disgusting, of mess, of pregnancy, that awful post-coital drip, of cystitis, of hard work for no reward, etc). I try to reframe it to myself, and I haven't been with anyone sexually abusive for almost 10 years, but that association is properly burned in deep and probably won't ever completely go away. So it's easier for me if we communicate more indirectly.
I didn't read it until later but I found the book Come As You Are by Emily Nagoski to be helpful too, although that was more about finding my own sexuality rather than seeing it as being the property of somebody else. TBH I am quite surprised that you haven't had more responses "getting it" - as I thought this was quite a common/normal experience and way to feel - clearly not